Projectors are widely used in many circumstances. Recently, with increasing development of science and technology, a pico projector has been introduced into the market. The pico projector is designed to have small size and light weightiness. The pico projector may produce projection images by different projecting technologies. For example, in a scanning projection system, a two-dimensional scanning mirror is used to periodically sweep a laser beam across a projection surface in order to produce the projection image on the projection surface.
FIG. 1A schematically illustrates the architecture of a conventional scanning projection system. As shown in FIG. 1A, the scanning projection system 100 comprises a laser module 151, a scanning mirror module 152, and a controlling circuit 153. The laser module 151 comprises plural color laser sources 122˜124 and plural optical alignment elements 155. The plural color laser sources 122˜124 are used for emitting plural color beams, respectively. By the plural optical alignment elements 155, the plural color beams from the plural color laser sources 122˜124 are mixed as a combined laser beam 154. Then, the combined laser beam 154 is reflected by the scanning mirror module 152, and projected on a projection surface 140. For example, the plural color laser sources 122˜124 are used for emitting a red beam, a green beam and a blue beam, respectively. Moreover, the scanning mirror module 152 is a microelectromechanical (MEMS) scanning mirror module.
Moreover, the controlling circuit 153 is used for outputting an image signal V to the laser module 151 and outputting a driving signal D to the scanning mirror module 152.
Please refer to FIG. 1A again. The swinging motion of the scanning mirror module 152 is controlled according to the driving signal D. Consequently, the projection points of the combined laser beam 154 are swept across the projection surface 140 by scanning each row of pixels from left to right and then from right to left and scanning rows from top to bottom. Generally, the start point of the scanning cycle is at an upper left corner of the projection surface 140, and the end point of the scanning cycle is at a lower right corner of the projection surface 140. According to the image signal V, the combined laser beam 154 with the corresponding image setting is projected on the corresponding scanning position during the swing of the laser module 151. After one scanning cycle is completed, a frame is displayed on the projection surface 140. Then, the projection point goes back to the start point (e.g. the upper left corner), and the next scanning cycle is performed to display the next frame.
Generally, the number of frames to be displayed every second is defined as a frame rate. For example, if the frame rate of the projection surface 140 is 60, it means that 60 scanning cycles are performed per second and 60 frames are continuously displayed on the projection surface 140 per second.
FIG. 1B is a schematic timing waveform diagram illustrating associated driving signals of the conventional scanning projection system. The driving signal D contains a fast-axis driving signal and a slow-axis driving signal. According to the fast-axis driving signal, the swinging motion of the scanning mirror module 152 along a fast-axis direction (e.g. a horizontal scanning direction or an x-axis direction) is correspondingly controlled. According to the slow-axis driving signal, the swinging motion of the scanning mirror module 152 along a slow-axis direction (e.g. a vertical scanning direction or a y-axis direction) is correspondingly controlled.
Please refer to FIG. 1B. At the time point t0, the scanning cycle of a first frame (frame1) is started. The time interval between two troughs of the fast-axis driving signal indicates one back-and-forth swinging motion of the scanning mirror module 152 along the horizontal scanning direction. The time interval between two troughs of the slow-axis driving signal indicates one back-and-forth swinging motion of the scanning mirror module 152 along the vertical scanning direction. Consequently, from the time point t0 to the time point t2, the first frame (frame1) is displayed on the projection surface 140. At the time point t1, the scanning cycle of a first frame (frame1) is ended. The time interval between the time point t1 and the time period t2 indicates the time period from the end point of the scanning cycle of the first frame (frame1) to the start point of the scanning cycle of a second frame (frame 2).
Similarly, the second frame (frame2) is displayed on the projection surface 140 from the time point t2 to the time point t3; and a third frame (frame3) is displayed on the projection surface 140 from the time point t3 to the time point t4.
As shown in FIG. 1B, the fast-axis driving signal may drive a fast-axis swinging motion of the scanning mirror module 152 along the horizontal scanning direction at a resonant frequency. Consequently, the scanning mirror module 152 is periodically swung in a sine-like wave form. Due to the sine-like fast-axis swinging motion, the projection points of the combined laser beam 154 are swept across the projection surface 140 at a non-constant velocity. The distance between every two adjacent projection points is not constant under the non-constant velocity, therefore the brightness of the frame is not uniform.
FIGS. 2 and 3 schematically illustrate the frame displayed on the projection surface of the conventional scanning projection system. As shown in FIG. 2, the distribution of the projection points at a left side 242 and a right side 244 of the projection surface 140 is denser, and thus the frame brightness presented at two side of the projection surface 140 is higher. Moreover, the distribution of the projection points at a middle region of the projection surface 140 is sparser, and thus the frame brightness presented at the middle region of the projection surface 140 is lower. In other words, the brightness values presented at the left side and the right side of the whole frame are higher, and the brightness value presented at the middle region of the whole frame is lower.
As shown in FIG. 1B, the slow-axis driving signal in the sawtooth wave form may drive the swinging motion of the scanning mirror module 152 in a periodic sawtooth wave form. However, due to the physical properties of the scanning mirror module 152, some drawbacks may occur. For example, when the slow-axis driving signal in the sawtooth wave form drives the slow-axis swinging motion of the scanning mirror module 152 along the vertical scanning direction, the scanning mirror module 152 may be suffered from jitter. Consequently, the frame brightness presented along the vertical scanning direction is non-uniform.
In particular, due to the physical properties of the scanning mirror module 152, the slow-axis driving signal fails to ideally drive the swinging motion of the scanning mirror module 152 at a constant velocity. Under this circumstance, the scanning mirror module 152 may be slightly suffered from jitter. Since the swinging motion of the scanning mirror module 152 is not ideally maintained at the constant velocity, some drawbacks may occur. For example, if the swinging speed is decreased, the distance between two adjacent scan lines is reduced, and thus the scan lines present bright. As shown in FIG. 3, if the swinging velocity of the scanning mirror module 152 along the vertical scanning direction and corresponding to a specified region 342 of the projection surface 140 is slower, the distribution of the scan lines at the specified region 342 of the projection surface 140 becomes denser. Consequently, the scan lines at the specified region 342 of the projection surface 140 present brighter than other region.
From the above discussions about the conventional scanning projection system, the brightness values presented at the left side and the right side of the whole frame are higher, and the scan lines at the middle region of the projection surface present brighter. Consequently, the user's eyes usually feel uncomfortable with the non-uniform brightness.
Due to the characteristics of the swinging motion or the characteristics of the driving signal, the projection points of the combined laser beam 154 are swept across the projection surface 140 at the non-constant velocity, and thus the distribution of the projection points are non-uniform. Moreover, if the optical path of the combined laser beam 154 to the projection surface 140 is adversely affected by other optical elements in the optical path, the projecting direction of the combined laser beam 154 is possibly shifted. That is, the positions of the projection points on the projection surface 140 are deviated. Under this circumstance, the distribution of the projection points on the projection surface 140 is not uniform. Consequently, the presented brightness of the frame on the projection surface 140 is not uniform.